1. Field of the Invention
The present invention relates to image tracking systems. More specifically, the present invention relates to techniques for automatically balancing channels to reject unwanted signals received by two color image detection systems.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Optical tracking systems process energy in the visible or infrared spectrum and generate signals indicative of track quality, the extent to which a target or guidance system is accurately tracking a desired object, image, or location. These optical image discrimination systems distinguish between different classes of objects based on emissions and reflections in certain spectral bands from discrete sources. In the process of discerning wanted from unwanted objects, conventional image discriminators presume certain differences in the spectral energy distribution of the two sources.
For example, to track a target which is reflecting sunlight, a conventional image discriminator would be equipped with a detector and one or more lenses. A reticle, such as the wagon wheel reticle of FIG. 1, would be moved in a nutation motion in relation to the image from the lens. The centered circle represents a point source at the track null, where a received image produces a constant carrier signal by passing sequentially across equal width opaque and transparent areas of the reticle. The offset circle represents a source displaced from this track null.
Typically, image nutation moves the image so that all points move in equal, in-phase circles. As the image crosses wide and narrow parts of the spokes, it produces a frequency modulation of the carrier signal, output by the detector, into one or more discrete frequency bands. The range of the frequency excursion represents the radial distance from the track null and the phase of the modulation envelope gives the direction of the displacement. The chief signal in a scene can therefore be centered, based on the phase and amplitude of the modulation, by moving the lens by use of a gimbal or other customary means. This signal is input to a tracker circuit typically implemented for such reticles with a frequency modulation (FM) discriminator. The spectral difference between the two frequency bands for sunlight would be readily made virtually identical and could be canceled by subtraction. For the target, however, the spectral difference between the two bands would be different. A discrete signal would result from the subtraction and could be used to center the system on the target image.
While this system works well when the spectral energy distribution of radiation from the unwanted object is constant or predictable, this system has some difficulty with images having varying or unpredictable spectral energy distributions. Such is the case, for example, when sunlight is reflected or scattered from surfaces of different spectral characteristics as these may change the spectral energy distribution of the received signal.
Unfortunately, conventional systems are designed to discriminate images with respect to a fixed spectral energy ratio. As a result, conventional systems are limited with respect to the variety of unwanted images which may be canceled.
Thus, there is a need in the art for a system which automatically enhances the ability of a system to discriminate between sources that can not be removed by a fixed cancellation scheme.